Sudden revelation (8)
I believe the answer is:
epiphany
'sudden revelation' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for epiphany that I've seen before include "Christian Festival, showing of Christ to the Magi" , "Christian festival of Magi visiting Chirst, celebrated on 6th January" , "Sudden realisation" , "Festival commemorating the showing of Christ to the Magi" , "A devine manifestation" .)